An injection molding technology to mold a meniscus-shaped plastic spectacle lens is shown in Japanese Patent Publication No. Hei 5-30608. In this technology, a cavity for molding the lens is formed inside an injection molding assembly, the cavity containing a pair of cavity forming members for shaping a convex surface and a concave surface of the lens disposed vertically opposite with each other. The injection molding assembly is heated before filling a molten resin in the cavity and one cavity forming member is moved toward the other cavity forming member to pressurize the molten resin filled in the cavity. Subsequently, the injection molding assembly is cooled to cool and solidify the molten resin, and the molded lens is taken out (=eject).
It is shown in Japanese Patent Laid-open No. Hei 6-31785 that an injection molding assembly is heated by means of a heating fluid such as steam and cooled by means of a cooling fluid such as air, water. In addition, after a molten resin is filled in a cavity in the injection molding assembly of which the temperature is raised beyond flow halting temperature of the molten resin, the temperature of the injection molding assembly is lowered below a glass transition point for molding a lens by cooling and solidifying the molten resin.
A lens is a precise molded product which requires high molding precision. Especially in a meniscus lens used for a spectacle lens, it is important that a convex shape and a concave shape of a pair of cavity forming members for shaping a convex surface and a concave surface of the lens are precisely transferred to the lens. However, when a lens to be molded has a difference in thickness between a central portion and a peripheral portion thereof, a thickness of the central portion being larger than that of the peripheral portion, the lens is easy to bend at the thin central potion. When such a disadvantage occurs, a high-precision lens to which a convex shape and a concave shape of cavity forming members are accurately transferred is not obtained.
For manufacturing a high-precision lens, it is important to prevent heat distortion or shrinkage deformation from occurring, which requires that the entire molten resin filled in the cavity is uniformly cooled. However, since the amount of the molten resin filled in the cavity corresponds to the volume of the lens and differs depending on a type of the lens, especially lens power, uniform cooling is difficult by controlling the temperature uniformly. Thus, temperature control of an injection molding assembly is desired for molding each lens highly precisely irrespective of the above difference.
An object of the present invention is to provide a plastic lens injection molding method to mold a high-precision lens by means of proper temperature control of an injection molding assembly.